This invention relates to combustion control systems, and more particularly to a diagnostic system for such combustion control systems.
Oxygen and combustibles analyzers are often used to analyze combustion gases in data collection and combustion control systems. These analyzers are effective in improving product quality and combustion efficiency as compared to systems without combustion gas analysis. A major drawback to these analyzers, however, is that the sampling, sensing, and heating systems required for their proper operation tend to be complex and prone to failure.
Current oxygen and combustibles analyzers provide little in the way of diagnostics to aid in system maintenance. As a rule, signals proportional to the oxygen and combustibles concentration in the analyzed gas are the only system outputs. Signals which represent internal analyzer condition are not provided as outputs. Correction factors based on the results of sensor calibration by automatic calibration systems are sometimes available. However, these correction factors are not easily accessible, and their meaning is not obvious to those without specialized knowledge.
These factors cause difficulty in interfacing the analyzer to a system controller. Because the basic operating conditions within the analyzer are unknown, erroneous flue gas measurements resulting from analyzer fault conditions may be used as control system inputs. Such erroneous inputs may result in inefficient and potentially unsafe conditions as the controller responds to them. A combustion control system using the output of a combustion analyzer as input could operate for significant periods of time based on false inputs. Thus, redundant measurements or complicated control systems were required. Thus, it would be desireable to have a combustion control system which monitors analyzer parameters and responds to abnormal conditions to default immediately.
A further consideration is the effects of electromagnetic and/or radio frequency interference (EMI/RFI) on control circuit operation. Such interference is generated by sources such as powerlines, rotating machinery, ignition systems, television and radio receivers, fluorescent lights, power amplifiers and transmitters of all types. Present combustion control systems generally comprise several packages each of which is environmentally sealed. Typically, the operator interface is located within the cabinet which houses the electronic control and processing circuitry and is accessed by a door. The only EMI/RMI protection is provided in the analyzer package to protect the low signal levels of the sensors and thermocouples. Typically, such protection is accomplished by shielding the cables which interconnect the several packages of the system by providing individual filters at the entrance of the housing for the electronic circuitry, and by providing conductive rubber gaskets on the door access for the housing. It would be desireable to have an improved housing for electronic control and processing circuitry of a combustion control system which minimizes its response to radio frequency and/or electromagnetic interference.